2.8's Grease Pencil: Is it possible to scale several frames at the same time?
Multiframe editing

Make sure to have both the Dope Sheet and the Viewport visible, then


in the Viewport, Tab into Edit mode
in the Dope Sheet, box select (B) the keyframes you want to be affected (they will turn yellow):

 
in the Viewport, make sure that the button next to "Multiframe" () is selected in the top bar
Make your edits


As of today, you won't be able to preview the effect on the other keyframes (except the current frame) while editing. But going back into object mode will reveal that the edit has taken place for all of them.